This invention relates to the manufacture of carbon-carbon brake discs. More particularly, this invention relates to brake discs which are coated with antioxidant compositions during the course of their preparation for use in braking systems.
Carbon-carbon brake discs are widely used on commercial and military aircraft. Wide-bodied commercial jets required improved brake materials because traditional steel brake systems simply could not absorb all of the thermal energy created during stops associated with landings. Carbon-based composites were developed which provide heat capacity, thermal conductivity, and thermal strength able to meet the demanding conditions involved in landing big jets. On the military side, the lower weights as well as the thermal and strength properties of the carbon composites has helped to ensure their acceptance in brake applications.
The production of carbon-carbon composite materials, including brake friction materials, has been described extensively in the prior art. One commonly used production method comprises molding a carbon fiber composite with a carbonizable resin, e.g. a phenolic resin, carbonizing the composite xe2x80x9cpreformxe2x80x9d, and then densifying the resulting porous material using chemical vapor infiltration (CVI) and/or resin impregnation processes. Another method comprises building up a fiber preform with textile materials and subsequently densifying the preform using a CVI process. Different structural types of carbon (graphitic, glassy, and pyrolytic) comprise the brake disc, which is somewhat porous. Further densification can be accomplished with, e.g., furfuryl alcohol infiltration or through incorporation into the carbon matrix of ceramic additives via infiltration with colloidal ceramics and their subsequent conversion to refractory materials.
Carbon-carbon brake disc friction performance is dictated by the carbon microstructure which arises from the manner in which the brake disc is manufactured. The amount of graphitization, for instance, can dramatically affect frictional and wear properties. Overall brake performance is particularly affected by the individual components, including fibers and types of matrix materials, at the friction surface.
One source of problems with these carbon composites is that they have low resistance to oxidation, by atmospheric oxygen, at elevated temperatures, that is, temperatures of 500xc2x0 C. (932xc2x0 F.) or higher. Oxidation not only attacks the surface of the carbon-carbon composites but also enters pores that invariably are present in such structures and oxidizes the carbon fibers adjacent to the pores and surfaces of the pores, thereby weakening the composites.
Exterior surfaces of carbon-carbon composites are therefore sometimes coated with a ceramic material such as silicon carbide to prevent entry of oxidizing agents such, as molecular or ionic oxygen from the atmosphere, into the carbon-carbon composites. Silicon carbide and other antioxidant coatings are described in detail in U.S. Pat. No. 4,837,073. The exterior surfaces of carbon-carbon composites may be, alternatively, coated with a glass-forming seal coat such as a boron or boron/zirconium substance. Borate glasses have also been used from the protection of carbon-carbon composites against oxidation. U.S. Pat. No. 5,208,099 describes antioxidant coatings that are formed from a SiO2xe2x80x94B2O3 gel and/or sol having a SiO2:B2O3 molar composition of 60-85:40-15. Borate glass antioxidant compositions are moisture-resistant and oxidation-resistant coatings composed of 40-80 weight-% B2O3, 5-30 weight-% SiO2, 7-20 weight-% Li2O, and 7-10 weight-% ZrO2 are described in detail in U.S. Pat. No. 5,298,311.
U.S. patent application Ser. No. 09/518,013 (Golecki), filed Mar. 3, 2000, relates to carbon fiber or Cxe2x80x94C composites that are useful in a variety of applications. Golecki teaches methods of protecting such composites against oxidation by coating them with fluidized-glass type mixtures. The fluidized-glass mixtures are maintained as liquid precursors and are applied to components formed of carbon fiber or Cxe2x80x94C composites. Once coated with the precursors, the coated Cxe2x80x94C components are heat-treated or annealed for one or more cycles through a series of gradual heating and cooling steps. This creates glass coatings having thicknesses of about 1-10 mils. The thicknesses of the glass coatings may be varied by varying the composition of the fluidized glass precursor mixtures, the number of application cycles, and/or the annealing parameters.
The Golecki application teaches that the fluidized glass materials may comprise such materials as borate glasses (boron oxides), phosphate glasses (phosphorus oxides), silicate glasses (silicon oxides), and plumbate glasses (lead oxides). These glasses may include phosphates of manganese, nickel, vanadium, aluminum, and zinc, and/or alkaline and alkaline earth metals such as lithium, sodium, potassium, rubidium, magnesium, and calcium and their oxides, and elemental boron and/or boron compounds such as BN, B4C, B2O3, and H3BO3. By way of example, Golecki discloses a boron-containing liquid fluidized glass precursor mixture that includes 29 weight-% phosphoric acid, 2 weight-% manganese phosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10 weight-% boron, and 55 weight-% water.
U.S. patent application Ser. No. 09/507,414 (Walker and Booker), filed Feb. 18, 2000, likewise relates to carbon-carbon composites and graphitic materials. The Walker and Booker application has as objectives the protection of carbon-carbon composites or graphites at elevated temperatures up to and exceeding 850xc2x0 C. (1562xc2x0 F.) and the reduction of catalytic oxidation at normal operating temperatures. Walker and Booker achieve these objectives by employing a penetrant salt solution which contains ions formed from 10-80 wt % H2O, 20-70 wt % H3PO4, 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate, and up to 2 wt % B2O3. Their penetrant salt solutions also include at least one of MnHPO4.1.6H2O, Al(H2PO4)3, and Zn3(PO4)2, in weight-percentages up to 25 wt %, 30 wt %, and 10 wt %, respectively.
The present invention provides, in one embodiment, a brake disc comprising a carbon composite body, wherein the surface of the brake disc is at least partially covered by a layer of an antioxidant composition that is readily visible even after charring. The brake disc of the present invention may be processed, before being incorporated into a brake system, to remove antioxidant composition that covers the working surface of the disc.
Another embodiment of the present invention is an antioxidant coating composition that is made up of from 10-75 wt % H2O, 20-65 wt % H3PO4, 0.1-20 wt % alkali metal mono-, di-, or tri-basic phosphate, 0-2 wt % hydrated boron oxide, 0-18 wt % KH2PO4, 3-10 wt % of a transition metal oxide, and 0-20 wt % hydrated manganese phosphate, 0-25 wt % Al(H2PO4)3, and 0-10 wt % Zn3(PO4)2, provided that at least one of Al(H2PO4)3, Zn3(PO4)2, and hydrated manganese phosphate is present.
A preferred compositional embodiment of the present invention is made up of from 20-25 wt % H2O, 40-45 wt % H3PO4, 10-15 wt % aluminum dihydrogen phosphate, 1-2 wt % H3BO3, 8-12 wt % KH2PO4, 2-5 wt % hydrated manganese phosphate, 2-5 wt % Zn3(PO4)2, and 4-7 wt % of either TiO2 or CoCr2O4, with the latter being especially preferred.
This invention also provides a brake disc made from a carbon composite article. In accordance with this invention, the surface of the article is treated with an antioxidant coating which contains from about 3 through about 10 weight-% of a transition metal oxide, and that transition metal oxide is selected so that the antioxidant coating (a) protects the carbon composite against oxidative weight loss, (b) does not abrogate friction properties of the composite, and (c) remains visible after charring.
This invention provides, in another of its aspects, methods for protecting carbon composite friction articles against oxidative weight loss. These methods of the invention may include preliminary steps of configuring carbon composite friction articles as a brake discs, steps of treating the surfaces of the composite articles with an antioxidant coatings that contain from about 3 through about 10 weight-% of a transition metal oxidexe2x80x94wherein the antioxidant coatings do not abrogate friction properties of the composite and wherein the antioxidant coatings remain visible after charring at 1600xc2x0 F. for 6 hoursxe2x80x94and subsequent steps of removing antioxidant coating from working surfaces of the brake discs.